The present invention relates to steering systems of the class in which manually-operable steering control means, with servo assistance, effect control of rotatable means to move a steering member so as to exert a steering effect. While not so limited, in one specific form it relates to servo-assisted systems for steering a boat.
It is known to provide servo assist of steering operations so as to reduce the manual power required for steering. This has been accomplished in the past by using motor means, commonly known as a servo motor, to drive rotatable means which control the motor of the steering member, and to control the starting, stopping and direction of motion of the servo motor by means of electrical spring-contacts associated with the wheel or other manually-operable steering control means. It is also known to provide direct manual steering in combination with the servo assist, for example by means of a rotatable flexible shaft coupling the steering wheel or the like to the same rotatable member as is driven by the servo motor.
However, in such a system it is possible for the servo system to breakdown in a manner which not only prevents proper servo assist, but also interferes with proper operation of the direct manual steering portion of the system. For example, if an electrical servo motor is used and the servo motor breaks down and becomes locked, it can lock the rotatable member against rotation and thus prevent direct manual steering also. All steering control is then lost.
In one known form of direct manual steering system, the rotatable member driven by the rotatable flexible shaft may comprise the nut or screw of a ball screw cylinder, i.e. a device comprising a nut, a screw which is rotatable relative to the nut, and a number of balls arranged between the nut and the screw in a closed loop. Rotation of the nut or screw is converted to linear motion of an output member which operates the rudder or other steering member. Such a ball screw cylinder device is disclosed, for example in Ser. No. 661,812 of Andersson. It is known to use such ball screw cylinders in steering systems using hydraulic servo assist mechanisms, but such hydraulic systems have required special piston and valve arrangements operated by the steering wheel or other steering control means, and normal ball screw cylinders have not been readily usable in such applications. Hydraulic servo assisted steering systems are therefore relatively complicated and expensive.
It is also known to drive a conventional, relatively inexpensive ball screw device with an electric motor, particularly with the ball screw directly actuated by the motor, as in devices for maneuvering aircraft landing gear or wing flaps, for example.
It is an object of the invention to provide a new and useful servo assisted steering system in which manual steerability is retained in the event of breakdown of the servo assist portion of the system.
Another object is to provide such system in which the servo motor is an electric motor and, preferably, both the motor and the manually-operable steering control means drive a conventional ball screw device to move the steering member.
Still another object is to provide such a steering system which is readily combinable with an additional manually-operable steering control means and/or with automatic steering apparatus.